1. Field of the Invention
The invention broadly relates to lasers. More particularly, the invention relates to circuits for powering laser diodes in low noise applications.
2. Brief Description of the Prior Art
All lasers use the principle of amplification of electromagnetic waves by stimulated emission of radiation. The term laser is an acronym for light amplification by stimulated emission of radiation.
The process of stimulated emission can be described as follows. When atoms, ions, or molecules absorb energy, they can emit light spontaneously (as in an incandescent lamp) or they can be stimulated to emit by a light wave. If a collection of atoms is pumped so that more are initially excited than unexcited, then an incident light wave will stimulate more emission than absorption, and there is a net amplification of the incident light beam. This is the way a laser amplifier works.
A laser amplifier can be made into a laser oscillator by arranging suitable mirrors on either end of the amplifier to form a resonator. Thus, the essential parts of a laser oscillator are an amplifying medium, a source of pump power, and a resonator. Radiation that is directed straight along the axis bounces back and forth between the mirrors and can remain in the resonator long enough to build up a strong oscillation. Radiation may be coupled by making one mirror partially transparent so that part of the amplified light can emerge through it.
The fundamental light-producing mechanism in an injection diode laser is the recombination of excess conduction-band electrons and valence-band holes. This recombination operation takes place by pumping excess carriers across a junction. That is, excess electrons are injected from a semiconductor n-layer and excess holes from a semiconductor p-layer into an active waveguide region, where they recombine, via stimulated emission, producing the desired gain. The lasing threshold is reached when optical loss is balanced by optical gain. Thus, lasing is driven by current.
Laser diodes are used in a variety of applications including data storage and retrieval with optical disks and magneto-optical disks. In these applications, laser light is reflected from the surface of a rotating disk. Depending on various ambient factors such as temperature, humidity, disk speed, and the condition of the surface of the disk, an element of noise may be coupled to the reflected light. In order to minimize noise, it is common to utilize an RF oscillator in conjunction with the laser current driver so that the laser light oscillates at a frequency which is much higher than that of the data.
There are several disadvantages to using an RF oscillator. These include low efficiency and unwanted high frequency radiation. The oscillators are inefficient because an inductor is required to decouple the current source so that only the laser is driven. Approximately 50% of the current is thereby wasted. The RF interference can interfere with the operation of the laser as well as nearby electrical components. Consequently, the RF oscillator must be mounted on the back of the laser diode package. Laser diodes driven by RF oscillators must also be certified by the Federal Communications Commission (FCC) for use in the United States. This can be costly and time consuming.
Self-pulsated lasers have been used as an alternative to using an RF oscillator in applications requiring only low power lasers. However, self-pulsating lasers are usually even less efficient than the RF oscillator and laser arrangement. Moreover, self-pulsating lasers generate undesirable heat.
It is therefore an object of the invention to provide a circuit for driving a laser diode.
It is also an object of the invention to provide a circuit for driving a laser diode which is highly efficient.
It is another object of the invention to provide a circuit for driving a laser diode which has low RF radiation emission.
It is still another object of the invention to provide a circuit for driving a laser diode which is low in cost.
It is yet another object of the invention to provide a circuit for driving a laser diode which has fewer components than conventional circuits.
It is also an object of the invention to provide a circuit for driving a laser diode which does not need to be certified by the FCC.
It is another object of the invention to provide a circuit for driving a laser diode which can be mounted very close to the laser diode.
It is yet another object of the invention to provide a circuit for driving a laser diode which does not generate undesirable heat.
It is still another object of the invention to provide a circuit for driving a laser diode which causes the laser to have a duty cycle greater than 50% and yet still operate in a multiple wavelength mode.
It is yet another object of the invention to provide a circuit for driving a laser diode which maintains a constant total current.
It is also an object of the invention to provide a circuit for driving a laser diode which can be mounted in die form very close to the laser diode.
It is another object of the invention to provide a single integrated circuit package which contains the laser diode and the circuit for driving the laser diode.
In accord with these objects which will be discussed in detail below, the circuit for driving the laser diode according to the present invention includes a fixed current source, a programmable current source, a differential output driver, a phase splitter, and a ring oscillator. The fixed current source is always coupled to the laser diode and supplies a few milliamperes (well below the current required to make the diode lase) in order to maintain forward voltage on the laser. The programmable current source is alternately coupled to the anode and the cathode of the laser diode by the differential output driver in response to signals from the phase splitter. The phase splitter changes phase at a frequency controlled by the ring oscillator.
The ring oscillator is made from an odd number of current inverters coupled in a loop. One of the current inverters is provided with hysteresis which is used to provide duty cycle control by making the previous current inverter have a different slew rate for high to low transition than from low to high transition. According to a presently preferred embodiment, a 25% duty cycle in the ring oscillator keeps the laser diode ON for 75% of the time. The frequency of the laser diode is nominally 300-500 Mhz.
The phase splitter provides two output signals which are substantially 180 degrees out of phase with each other. When one is high, the other is low and they change at exactly the same time in response to the output of the ring oscillator.
The differential output driver includes two switching transistors which are driven (via their gate connections) by the outputs of the phase splitter such that one transistor is OFF when the other is ON and they change at exactly the same time. More particularly, the gate of the first transistor is coupled to the first output of the phase splitter and the drain of the first transistor is coupled to the anode of the laser diode. The gate of the second transistor is coupled to the second output of the phase splitter and the drain of the second transistor is coupled to the cathode of the laser diode. The sources of both transistors are coupled to the programmable current source.
According to a presently preferred embodiment, the circuit of the invention is implemented as an integrated circuit chip which is mounted in die form very close to the laser diode in a leadframe package. The very short connections between the circuit and the laser diode minimize RF emissions. According to this embodiment, the leadframe package also includes all of the photodetectors needed for using the laser diode to read disks.